1. Field of the Invention
The present invention relates to a process for producing an oriented inorganic crystalline film having a layered crystal structure, and the oriented inorganic crystalline film. In addition, the present invention also relates to a semiconductor device (such as a thin-film transistor (TFT)) using the oriented inorganic crystalline film.
2. Description of the Related Art
Recently, various flexible devices have been receiving attention. The use of the flexible devices is widely spread, and the flexible devices include, for example, electronic paper, flexible displays, and the like.
Conventionally, the flexible devices, as well as the devices using glass substrates, are mainly manufactured by vacuum film formation (such as sputtering or vacuum evaporation) and photolithographic patterning. However, the vacuum processes for the vacuum film formation and the photolithographic patterning are complex and entail high equipment cost. In addition, since unnecessary portions of uniformly formed films are removed, considerable amounts of materials are wasted.
Therefore, recently, the liquid phase techniques have been receiving attention because of the simplicity and the ability to reduce the cost. According to the liquid phase techniques, thin films are formed by using raw-material solutions containing the components of the thin films. The liquid phase techniques include, for example, application and printing (such as inkjet printing and screen printing). The liquid phase techniques do not need vacuum processes. In addition, since the patterning can be performed by direct imaging, the liquid phase techniques enable simple manufacture of devices at low cost.
For example, manufacturing of a TFT (to be mounted in a flexible display device) by a liquid phase technique is currently studied. Since the flexible devices use resin substrates having lower thermal resistivity than the inorganic substrate such as the glass substrate, the entire process of manufacturing a flexible device using a resin substrate is required to be executed under the thermal-resistance temperature of the resin substrate. Therefore, in the case where a TFT is produced by a liquid phase technique, use of organic materials for semiconductor layers is mainly studied. The thermal-resistance temperature of the resin substrate is normally 150 to 200° C., although the thermal-resistance temperature depends on the material. Even the thermal-resistance temperatures of the thermally resistant materials such as polyimides are approximately 300° C. at the highest.
Nevertheless, since the organic semiconductors tend to be inferior to the inorganic semiconductors in performance and durability, it is desirable that TFTs be formed of inorganic materials on a flexible substrate by use of a liquid phase technique. A known technique for forming an inorganic film by a liquid phase technique uses the sol-gel process. However, when a film is formed by use of the sol-gel process, the type and the orientation of a substrate underlying the film is reflected in the characteristics of the film. Since the flexible substrates are normally amorphous, an inorganic film formed immediately on a flexible substrate does not exhibit orientation.
In order that an inorganic film has satisfactory semiconductor characteristics, it is desirable that the inorganic film has crystalline orientation. For example, the electron mobility μ of a (001)-oriented film of an In—Ga—Zn—O-based material formed by vapor phase deposition is reported to be approximately 80 cm2/VS, which is approximately eight times the electron mobilities of the amorphous or polycrystalline film of an In—Ga—Zn—O-based material.
Japanese Unexamined Patent Publication No. 5 (1993)-000897 (hereinafter referred to as JP5-897A) discloses a process for forming an orientated film of LiNbO3 on a base crystal by use of the sol-gel process, and a composite crystal produced by the process.
According to the technique disclosed in JP5-897A, the orientated film of LiNbO3 is required to be formed on a substrate called a base crystal, and monocrystalline plates of sapphire, magnesium oxide, and the like are indicated as examples of the base crystal in JP5-897A. In addition, JP5-897A reports, that the film obtained by the above technique has the same face orientation as the base crystal.
Since most of the flexible substrates used in flexible devices are amorphous substrates as explained before, it is impossible to use the technique disclosed in JP5-897A in production of flexible devices. In the case where an inorganic film is formed on a flexible substrate through a base crystal as disclosed in JP5-897A, the inorganic film has the same orientation as the base crystal. However, in this case, the structure is complex, and the increased thickness can limit the flexibility.
Further, even in devices other than the flexible devices, it is also desirable that an oriented film be easily formed on an inexpensive amorphous substrate (such as a glass substrate) by using a liquid phase technique.